Air-augmented rocket propulsion is a means for very substantially increasing the range or payload of a rocket vehicle whose trajectory is confined within the earth's atmosphere. Such vehicles comprise a booster motor or stage wherein a conventional stoichiometric solid propellant accelerates the vehicle to velocities adequate to force ram air at sufficiently elevated pressure and temperature into the afterburner stage for combustion of combustible products produced by combustion of the fuel-rich propellant in the sustainer stage. By stoichiometric is meant a propellant oxidizer content sufficient at least to oxidize all of the available carbon in the propellant binder to carbon monoxide and any other high-energy fuel additive, such as metal, substantially fully to its stable oxide. Available carbon means carbon not already oxidized or oxidizable by an oxidizing element molecularly combined in the organic binder.
A second or sustainer stage comprises a combustion chamber containing a fuel-rich solid propellant, namely a propellant containing sufficient available oxidizer component to sustain combustion of the propellant, but insufficient to oxidize the high-energy boron fuel additive after substracting the oxidizer requirements of the organic binder for oxidation of its available carbon to carbon monoxide. The term "binder" includes, in addition to an organic polymer, organic additives, such as plasticizers, burning rate catalysts, stabilizers, dispersing agents, and the like, which contain carbon available for combustion. Accordingly, when such a fuel-rich propellant burns, a major proportion of the high-energy additive is ejected with the combustion products of the fuel-rich propellant as free boron and generally, small amounts of its stable and lower metal oxides, the latter of which are then available for combustion downstream of the propellant in an afterburner combustion zone or stage into which ram air is introduced. Additionally, other underoxidized or unoxidized products, such as CO, H.sub.2 and C, are also available for combustion in the afterburner stage. The high temperature, high pressure combustion products vent through a rocket nozzle to produce thrust.
Such fuel-rich, solid propellants per se are low performance propellants and have substantially lower specific impulse than conventional, stoichiometric propellants, but this is greatly counterbalanced by the combustion in the afterburner, which can contribute additional specific impulse of as much as 600 to 900.
The increased air-augmented vehicle range (or conversely higher payload for a given range) is achieved by very substantial weight reduction in the oxidizer component of the sustainer propellant and the utilization of ram air to complete combustion of ejected unoxidized or incompletely oxidized combustion products in the afterburner.
Since an air-augmented rocket system is more complex than a conventional rocket because of its requirement for ram air ducts and stable afterburning, successful performance trade-off requires a fuel-rich propellant that burns stably and efficiently to provide maximum quantities of combustible after products, particularly free, high-energy metal fuel, in a form which will burn efficiently and completely in the afterburner. Thus the formation of combustion product residues, such as metal oxides in the form of slag, which encase the free metal and prevent its injection into the afterburner in readily combustible form, must be minimized. Boron is a preferred metal in air-augment propulsion because of its very high heat of combustion per unit mass and per unit volume. However, ejection problems can be particularly severe in the case of boron. Development of boron-fuel-rich propellants having high ejection efficiency has posed difficulties.
In-flight controllability of the degree of thrust on command is a very desirable feature in an air-augmented rocket system since it permits flexibility in speed, manueverability, trajectory, and range. To be effective, response time of the rocket must be rapid. An effective way to achieve controllability is by using a boron-fuel-rich propellant having a relatively high pressure exponent (n). The higher the pressure exponent, the greater is the rate of burning rate increase for a given increase in combustion chamber pressure. Thus for a relatively small range of combustion chamber pressures, produced, for example, by a pintle nozzle on command, substantial and rapid increase or decrease in burning rate and, therefore, in thrust, can be achieved. It has, however, been difficult to produce boron-fuel-rich propellants having relatively high pressure exponents.
Agglomerate particles of metal fuel and of metal fuel oxidizer and burning rate catalyst have been suggested by U.S. Pat. Nos. 3,133,841 and 3,454,437 for use in conventional essentially stoichiometric propellants wherein the function of the metal fuel component is primarily to burn as a component part of the propellant per se and thereby to increase the specific impulse and performance of the propellant. In U.S. Pat. No. 3,133,841 agglomeration of small metal particles is recommended to reduce pyrophoricity and thereby handling problems. In U.S. Pat. No. 3,454,437, a blend of metal, oxidizer, and burning rate catalyst are agglomerated in order to increase the amount of oxidizer that can be introduced into the propellant, and, thereby, increase the per se performance and specific impulse of the conventional propellant. U.S. Pat. No. 3,377,955 is concerned with the problem of incorporating exotic, highly reactive fuels, such as lithium hydride, into a propellant and resolves this by providing fuel cores, such as pharmaceutical-size tablets comprising the reactive fuel, preferably with a non-reactive coating. They do not address themselves to low-performance fuel-rich propellants for use in air-augment propulsion systems wherein a major objective is efficient ejection of boron by the propellant into an afterburner stage or the particular problems attendant thereto. Boron per se, like aluminum, is not an exotic or highly reactive fuel and can be readily incorporated in powder form into a propellant composition without posing problems of reactivity.
The objective of the invention is to provide improved boron-fuel-rich propellant compositions, for use in air-augmented rocket motor systems, which are characterized during combustion by increased ejection of free boron for combustion in the afterburner zone and which thereby improve total performance of the systems.
Another object is to provide improved boron-fuel-rich propellants, for use in such systems, which are characterized by substantially increased pressure exponents.
Still another object is to provide improved boron-fuel-rich propellants characterized by relatively high pressure exponents which, thereby, are particularly useful in controllable air-augment rocket propulsion systems.